Differentiation of seminomatous from nonseminomatous testicular tumors with MR imaging.

Distinguishing seminomatous from nonseminomatous testicular neoplasms preoperatively is useful because treatment of these two tumors types is different. We evaluated whether the distinction could be made with MR imaging in six patients with seminomatous and nine patients with nonseminomatous testicular tumors (including teratoma, teratocarcinoma, embryonal cell, and choriocarcinoma). The MR diagnoses, which were obtained from the formal reports of the MR studies done and interpreted before orchiectomy, were compared with the pathologic diagnosis. The distinction between the two tumor types on MR images was based on the signal intensity and heterogeneity of the lesion. We also retrospectively compared the MR findings with tissue histology. MR scans in nonseminomatous tumors showed a marked heterogeneous mix of signals, with some regions less intense and others more intense than normal testicular tissue on both proton-density and T2-weighted images. The typical background signal was nearly equal to normal testicular tissue. The tumors also had a dark band at their periphery that correlated with a fibrous tumor capsule on histologic examination. In contradistinction, seminomatous tumors were isointense with testis on proton-density images and consistently hypointense and relatively homogeneous on T2-weighted images. These tumors typically lacked a capsule on MR images. One lesion, only 3 mm in diameter, could not be characterized on MR images because of limitations in spatial and contrast resolution. In 13 of the 14 lesions that could be characterized, the histologic type was predicted correctly on the basis of the MR appearance. The one error occurred in a patient with pure seminoma. Although the MR appearance of the lesion was otherwise similar to the other seminomas, the lesion had a single focus of bright signal that was due to hemorrhage. This focus was interpreted incorrectly as a nonseminomatous element. Our findings in this small series of patients suggest that MR imaging can be used to distinguish seminomatous from nonseminomatous testicular tumors.